A sensor is described, for example, in D1 (U.S. Publication No. 2006/0124976) and is shown in FIG. 1. The sensor of D1 is made on a P− type substrate 100 and comprises a gate 102, a pinning layer 106 strongly doped with P+ type impurities, an accumulation region 108 doped with N type impurities and situated beneath the region 106, a read region 110 strongly doped with N type impurities. The length of the read region 110 is bounded on one side by the gate 102 and on the other side by an insulating border 112; the region 110 forms a floating drain of the transistor. The regions 106, 108 are bounded on one side by an insulating border 114 and on another side by the gate 102; the regions 106, 108 form two PN junctions of the photodiode with the substrate 100. The accumulation region 108 forms both the volume channel of the photodiode and the source of the transistor.
The working of such a sensor includes an accumulation phase and a transfer phase. During the accumulation phase, a ground potential is applied to the substrate 100 and to the pinning layer 106 and a reverse bias potential Vd is applied to the accumulation region 108; when photons strike an upper surface 118 of the diode, electron-hole pairs are generated in the diode, in the regions 108, 106 and 100 and the electrons are stored here in the N region 108. During the transfer phase, the gate 102 of the transfer transistor is turned on, a surface transfer channel is created beneath the gate 102 and the electrons generated by photoemission in the region 108 are transferred to the read region 110 whose content can then be read by appropriate read means (not shown in FIG. 1).
The depths of the regions 106,108 and the concentrations in N, P dopant of these regions are chosen so that, when the potential Vp is applied to the region 108, the space charge zones of the two PN junctions of the diode meet. Thus, during the accumulation phase, no majority carrier can be extracted from the region 108. Such a diode is called a pinned diode or a fully depleted diode.
In photodiode sensors, there often arises a problem of remanence due to a poor transfer of charges. This is due to the difference in position between the channel in volume of the photodiode (region 108) and the surface channel of the transistor situated beneath the gate of the transistor.
In the document D1, this problem is limited by making a gate 102 that is buried so that the transfer channel beneath the gate is in contact with the accumulation zone 108. The transfer of the charges is thus facilitated because the charges do not need to go in transit through the pinning zone 108.
However, the sensor of D1 is difficult to make. The different implantations of dopant impurities and the making of the gate indeed require the use of different masks, and this leads to an imprecise definition of the different regions or an encroachment by one region on the other or an encroachment by an implantation region on the gate.